1. Field of the Invention
The present invention relates to an acoustic resonator, and more specifically, to a film bulk acoustic resonator and a filter circuit used in high frequency bands.
2. Description of the Related Art
Recently, wireless communication systems such as mobile telecommunication devices, and high-speed data transfer wireless local area networks (LAN) use high frequency bands which exceed the GHz range. A film bulk acoustic resonator (FBAR) is used as a high frequency element in high frequency electronic equipment of these types of wireless communication systems.
In the past, bulk (ceramic) dielectric resonators, surface acoustic wave elements (SAW) have been used as resonators for high frequency bands. Compared to these resonators, the FBAR is better suited for miniaturization, and has attributes allowing the FBAR to respond better to even higher frequencies. Thus, there is continued development of high frequency filters and resonance circuits using the FBAR.
In a basic structure of a FBAR, a piezoelectric film, such as aluminum nitride (AlN) and zinc oxide (ZnO), is sandwiched between a first electrode and a second electrode, which are opposed to each other. A resonator of the FBAR is disposed above a cavity provided below the first electrode, in order to attain high performance. In general, the piezoelectric film has a larger area than the first and second electrodes.
The FBAR uses resonance of longitudinal waves that are transverse extension (TE) waves propagating through the piezoelectric film between the first and second electrodes. However, in the FBAR, the longitudinal waves are generated as the primary vibration mode, and parasitic transverse waves that are transverse shear (TS) waves, are also generated. The transverse waves generated in the resonator are reflected at end portions of the first and second electrodes and end portions of the piezoelectric film where mass densities of the FBAR change drastically. For example, when the geometry of the first and second electrodes is a square or a rectangle, the transverse waves which are reflected at the end portions of the first and second electrodes interfere with each other so as to generate spurious resonance. Thus, a spurious resonance in the resonant characteristic of the FBAR is generated due to a parasitic resonance induced by the transverse waves.
In order to control the spurious resonance in the FBAR, it has been proposed that the first and second electrodes are formed as an irregular polygon (refer to United States Patent Application Specification No. 006215375A) However, when the electrode geometry is an irregular polygon, it is difficult to reduce the area occupied by a FBAR element on a substrate. Thus, it is desirable to manufacture the FBAR element with an electrode geometry such as a square shape, a rectangle shape, a round shape, and an ellipse shape, in order to miniaturize the FBAR.